The invention relates to making an x-ray source suitable for submicron lithography of semiconductors in commercial quantities. The technical specification of such a source can be derived from a research paper "Intense Pulsed Plasma X-Ray Source for Lithography: Mask Damage Effects" published by H. A. Hyman et al. in the Journal of Vacuum Science Vol. 21, pp. 1012-1016 (1982). Essentially it requires that in order to avoid mask thermal damage a source with small size, high conversion efficiency of electrical energy into x-rays about 1 keV and x-ray emission during the required exposure time to be continuous or distributed over several small pulses rather than concentrated in one strong pulse.
The common most x-ray source is the electron impact type source in which electrons are accelerated by an electric field to impinge on a metal target. The electrical to x-ray energy conversion efficiency of such a source is so low that heat dissipation in an acceptable lithography source presents an insurmountable problem.
In another type of x-ray source, as shown by Nagel et al. U.S. Pat. No. 4,184,078, Jan. 15, 1980, a high power pulsed laser beam strikes a solid target to form dense plasma which emits x-rays for a very short duration. The laser required for such a source is very bulky and expensive and not economical to operate at a repetition rate desired for commercial x-ray lithography.
In yet another type of x-ray source, a gas is puffed between a pair of electrodes to form an annulus, and a capacitor bank discharged between the electrodes through the gas annulus, which then carries high current and as a result implodes on its axis. The implosion, being driven by the energy stored in the capacitor bank, forms a hot and dense plasma column causing x-ray emission. The main limitation of the device is the low repetition rate due mainly to the time required to pump down the puffed gas.
In yet another type of x-ray source, as shown by Cartz et al. U.S. Pat. No. 4,504,964, Mar. 12, 1985, high power pulsed laser beams are used to produce an annulus of material between two electrodes through which a high current is passed by discharging a capacitor bank. This causes the material to implode on the axis of the annulus, forming dense hot plasma which is then a pulsed source of x-rays. The use of a high power laser makes the device bulky and very expensive.
There are other types of imploding plasma x-ray sources developed for research such as the exploding wire array and the thin foil annulus, but these require replacement of imploding material after each shot and thus are usable for commercial x-ray lithography.
As has been shown by McCorkle U.S. Pat. No. 4,201,921, May 6, 1980, x-rays can be repeatedly produced by impinging an intense electron beam on a plasma generated by passing a high current along the inner wall of an insulator capillary. One of the problems with such a source is that the capillary needs to be replaced after several shots.